Spinal fixation systems

ABSTRACT

The present disclosure includes fixation devices that comprise one or more porous elements or fenestrations to aid in osteo-integration of the fixation device. These fixation devices may be additively manufactured using biocompatible materials such that the solid and porous aspects of the screw are fused together into a single construct. Spinal stabilization systems are also disclosed having spanning portions extending between and securable to pedicle screw assemblies, the spanning portions have stiffness characteristics that may be variable or selectively adjustable, and/or have non-linear behavior with respect to force versus distortion. Additionally, the systems may utilize a plurality of spanning portions in which two or more of the spanning portions have different stiffness characteristics. Methods for fabricating and using the foregoing devices are also described herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application and claims thebenefit and priority of U.S. patent application Ser. No. 15/675,104,filed Aug. 11, 2017, which is a continuation-in-part of U.S. patentapplication Ser. No. 15/416,975, filed on Jan. 26, 2017, which whichissued as U.S. Pat. No. 9,987,024 on Jun. 5, 2018, which claims thebenefit of U.S. Provisional Application No. 62/373,855, filed Aug. 11,2016.

This application is also a continuation in part and claims the benefitand priority of U.S. patent application Ser. No. 14/830,523 filed Aug.19, 2015, which is a continuation of U.S. patent application Ser. No.12/172,996 filed Jul. 14, 2008, now abandoned, which claims the benefitof U.S. Provisional Patent Application No. 60/959,456 filed Jul. 13,2007. The entire disclosures of the prior applications are incorporatedby reference.

FIELD OF THE INVENTION

The present disclosure relates to the field of medical devicesgenerally. More specifically, the present disclosure relates to dynamicfixation and stabilization devices for use in spinal-related surgeries.Systems and methods for fabricating and using the foregoing devices arealso disclosed herein.

BACKGROUND OF THE INVENTION

Individuals who suffer degenerative disc disease, natural spinedeformations, a herniated disc, spine injuries or other spine disordersoften require surgery on the affected region to relieve pain and preventfurther injury. Such spinal surgeries may involve fixation of two ormore adjacent vertebral bodies. For patients with varying degrees ofdegenerative disc disease and/or nerve compression with associated lowerback pain, spinal fusion surgery or lumbar arthrodesis (“fusion”) iscommonly used to treat the degenerative disease. Fusion commonlyinvolves distracting and/or decompressing one or more intervertebralspaces, followed by removing any associated facet joints or discs, andthen joining or “fusing” two or more adjacent vertebra together. Fusionof vertebral bodies also commonly involves fixation of two or moreadjacent vertebrae, which may be accomplished through introduction ofrods or plates, and screws or other devices into a vertebral joint tojoin various portions of a vertebra to a corresponding portion on anadjacent vertebra. Given the complexities of surgical procedures, aswell as anatomical variation between patients who receive surgicaldevices, it is often challenging to provide a device or implant thatachieves the needs of a particular patient without completelycustomizing the device or implant for a single patient.

Many prior art fixation devices suffer from significant disadvantages,such as poor stability, poor flexibility, poor accuracy, difficulty inhandling, lack of customized features, inability to combine with othermaterials, loss of fixation over time, subsidence and otherdisadvantages. Certain fixation devices also impair visibility andprovide little or no ability for the operator to gauge depth oraccuracy. These problems and shortcomings are even more noticeable forfixation devices used in surgical settings or which otherwise requireprecision.

In addition, fixation devices used in surgical settings can also sufferfrom further shortcomings. For example, pedicle screws are subject torelatively high failure rates, which is often attributed to a failure ofthe bone-screw interface. Screws for use in surgical settings may alsobe limited for use in only certain boney anatomies, or with only certaintypes of drilling apparatus, and may not be suitable for combinationwith other devices or materials.

Accordingly, there is a need for a fixation device that decreases themean time for affixing the device to the desired location, enhancesdepth control, stability and accuracy, and which otherwise overcomes thedisadvantages of the prior art. There is also need for a more customizedfixation device, such as an orthopedic screw, which includes one or moreporous elements or fenestrations to aid in osteo-integration whenimplanting the fixation device. The fixation device may be additivelymanufactured using biocompatible materials such that the solid andporous aspects of the device are fused together into a single solidconstruct, and potentially having the porous elements interdigitatedwithin and around various solid elements of the device.

The prior art also fails to teach a system for creating a customizedfixation device based on patient data, such as data derived from apatient's MRI or CT scan. For example, the availability ofpatient-specific data (for example, a vertebral body) may allow asurgeon to accommodate for subtle variations in the position andorientation of a screw or other fixation device to avoid particularboney anatomy, or irregularities in the positioning and alignment of theadjoining vertebral bodies. As another example, the use of patient datamay also assist a surgeon in selecting a desired trajectory for afixation device so as to avoid, for example, crossing the pedicle walland violating the spinal canal during a spine-related procedure. The useof patient-specific data permits the surgeon to avoid these types ofmistakes and may comprise specific orientation, end-stops/hard stops, orother safety related features to avoid over-torque or over-insertion ofthe fixation device. This data also permits the surgeon to quickly andefficiently locate and place devices with correspondingpatient-contacting surface(s), while ensuring the fixation device is inthe appropriate location and orientation.

Current spinal stabilization devices and systems typically involve theuse of anchors secured with a plurality of vertebrae and sometimes spanbetween the anchors. Such devices are often referred to by the portionof the vertebra to which the anchors secure. For instance, a laminarstabilization system utilizes anchors, typically hooks, secured with thelamina of a vertebra. As another example, a stabilization utilizinganchors in the form of a screw is often referred to as a pedicle screwsystem, as the screws themselves are driven into the pedicle portion ofthe vertebra. Generally speaking, the spanning member is the leastconsidered part of this type of system. A surgeon's choices for spanningmembers are virtually limited to selecting either a rod or a bar, thelength of the spanning member, and a cross-sectional dimension such asthe rod's diameter. It should be noted that there are particularizedtypes of rod a surgeon can select. Generally, however, these rods arelimited in use to an entire system, and the deviation from the standardrod provided by these rods is not for mechanical behaviorcharacteristics, instead being for cooperation with the otherparticularized features of a specific stabilization system.

Other than portions of the above discussion, the term “stabilizationsystem” is meant to refer only to spinal stabilization systems thatattach to one or more vertebrae in a manner that does not affect orinterfere with the intervertebral space, nucleus, or annulus.Accordingly, laminar or pedicle systems or the like are each intended tobe encompassed by the term “stabilization system.” In general terms, astabilization system is implanted through an open and retracted incisionby securing at least one anchor on an inferior vertebra and at least oneanchor on a superior vertebra. It should also be noted that the medicalcommunity is continuing to develop minimally invasive surgicaltechniques for implantation of such devices. Typically, a pair ofanchors is secured with each of the vertebrae, and typically thevertebrae are adjacent. In some forms, the stabilization system may spanthree or more vertebrae and be secured with any two or more of thevertebrae.

Spanning members are then secured with the anchors. This commonlyrequires forcing rods into a yoke secured with each of the anchors. Insome forms, the anchor and yoke are of a type referred to as “polyaxial”by their ability to pivot relative to each other so that a channel inthe yoke for receiving the rod becomes aligned in an optimal orientationfor receiving the rod. The spanning members are usually then secured inand with the yoke with a securement in the form of a cap that isreceived in an upper portion of the yoke channel.

The entire stabilization system is generally highly rigid. Once the rodis secured therein, the cap and the yoke frequently distort or defacethe surface of the rod via the pressure exert to secure the rod therein.This prevents movement of the rod within (such as rotation) or relativeto the yoke and anchor (such as longitudinal sliding). The rod itself isformed of a high modulus of elasticity metal, and its mechanicalbehavior displays little elasticity.

Stabilization systems have been developed to allow some motion in one ormore directions. Generally, motion of a normal, healthy spine includesanterior-posterior flexure, lateral flexure, and rotation, or anycombination of these. Due to disease, damage, or natural defect, thepurpose of the stabilization system may vary. Depending on such purposefor the stabilization procedure utilizing the stabilization systems,motion in one or more directions may be preferred to a rigid system.

It is also known that there are medical detriments that can arise fromfull immobilization. For instance, it is know that a lack of pressure(i.e., stress, or weight) on bones can result in a decrease in density.An expression known as Wolf's law describes the benefits of pressure onbones or bone fragments as they are healing, benefits that can benegated by an overly rigid spinal stabilization system. It is alsosuspected that intervertebral structures may suffer from a lack of useresulting from rigid systems. Additionally, full immobilization canresult in overstressing of adjacent areas, thus producing adjacentsegment degeneration.

Accordingly, some stabilizations systems have been designed to allow theportion of the spine to which the system is secured to bend itself. Forinstance, the ends of a spanning member may be curved relative to eachother due to motion in some directions, like a cylindrical rod beingcurved.

A complicated example of stabilization system permitting some bendingmotion is described in U.S. Pat. No. 5,961,516, to Graf. In simpleterms, the system of the '516 patent includes anchors for respectivevertebrae and a spanning structure connected with the anchors. Thespanning structure includes a ball joint between two portions, and a“compressible” body acting as a shock absorber. The various componentsof the system of the '516 patent must clamp tightly and utilize frictionin order to resist free movement. Over time, such friction results inwear to the components, which in turn may lead to reduced performance ofthe components and revision surgery, or fragments of the componentsbeing free in the patient's body. It is also known that implantation ofan elastomeric/polymeric compressible member is difficult as thematerial is prone to release of polymeric byproducts and is prone tochemical and mechanical degradation.

Another direction of motion that ideally is accommodated is that whichshifts the anchors themselves relatively and directly in line with thespanning structure. The '516 patent purports to provide a system thatallows spinal motion in all directions, only the compressible memberallows the spanning structure itself to shorten; additionally, thecompressible member is not shown as being able to expand for thespanning structure being lengthened.

Once implanted, the stabilization systems are generally constant intheir behavior characteristics, other than changes due to wearing ofcomponents or the like. To be specific, a surgeon may select a specificdiameter for a rod to span between two anchors, and the diameter andmaterial can be selected for their mechanical properties. The surgeonmay also determine either a length of the rod or a distance between theanchors based on how the rod is secured with the anchors. However, theselection of the rod diameter is quantized as it is a specific size, andthe surgeon is unable to adjust the exact diameter during a procedureother than to select from specific, predetermined diameters. Subsequentto the surgical procedure, the surgeon is unable to adjust the distancebetween the anchors without a further, revision surgical procedure,which would also be required if a surgeon were to determine a differentdiametrally-sized rod would be preferred (such as to increase ordecrease the flexure of the spanning structure).

In the selection of the stabilization systems discussed, a surgeon isnot provided with sufficient implant options for selecting a desiredamount of permitted motion. For instance, a surgeon's choice inimplanting a pedicle screw system is generally limited to thecross-sectional size of the rod spanning between the pedicle screwassemblies, and larger rods require a larger yoke provided on thepedicle screw for receiving the rod therein. Even using systems that aredesigned to permit some degree of motion, such systems do not provide asurgeon the ability to optimize the motion permitted based on aparticular patient, they do not allow a surgeon to adjust the mechanicalbehavior of the system through a linear range, and they do not allow asurgeon to adjust the mechanical behavior without full-scale revisionsurgery.

It would therefore be advantageous to provide a fixation device thatsignificantly reduces, if not eliminates, the shortcoming, problems andrisks noted above. As also understood from the foregoing, there is along-felt need for improved spinal stabilization systems. Otheradvantages over the prior art will become known upon review of theSummary and Detailed Description and the appended claims.

BRIEF SUMMARY OF THE INVENTION

According to various embodiments presented herein, the presentdisclosure describes a fixation device, such as a screw, comprising oneor more fenestrations which permit introduction of at least one othermaterial or substance to through the fenestrations in the fixationdevice. In other embodiments, the fenestrations permit the fixationdevice to capture and retain material. In yet another aspect of thepresent disclosure, a method of using the fixation devices describedherein is disclosed, including but not limited to in a surgical setting.

One particular aspect of the present disclosure involves a fixationdevice, such as an orthopedic pedicle screw or implant that ismanufactured such that it includes one or more porous elements orfenestrations in order to aid in osteo-integration of the implantedfixation device. For instance, a pedicle screw (as one type of fixationdevice) may be additively manufactured using biocompatible materialssuch that the solid and porous aspects of the screw are fused togetherinto a single solid construct with the porous elements interdigitatedwithin and around various solid elements of the screw.

In yet another aspect, a surgical screw design having at least a portionor section incorporating a porous structure enables bony ingrowththrough the porous section/portion of the screw, and thereby facilitatesbiocompatibility and improves mechanical characteristics. The porouselements of the screw may be designed to more closely resemble that ofthe patient's anatomy, in order to reduce discontinuities and stressrisers at the bone/screw interface. Bony ingrowth within one or moreporous elements of the screw in turn facilitates screw pullout strength,and may reduce the risk of loosening of a fixation device under dynamicloading situations.

In accordance with another aspect, an orthopedic device is disclosed toprovide stabilization of the spinal column between anchorage locationson a minimum of two vertebral bodies comprising structural member(s) orspanning portions between each anchorage point, the device or systemhaving the ability to provide stiffness, and the stiffness beingvariable in longitudinal and transverse planes relative to the spinalcolumn or vertebral bodies.

The stiffness of the structural member(s) can be varied by adjustment ofcross-sectional area properties. The stiffness of the structuralmember(s) can be varied by adjustment of helical coil springtension/compression. The stiffness of the structural member(s) can bevaried by adjustment of hydraulic pressure or volume. The stiffness ofthe structural member(s) can be varied by adjustment of pneumaticpressure. The stiffness of the structural member(s) can be varied bycombining materials of differing properties.

An orthopedic device of the present invention may comprise at least twostructural members, one of which has an outer cross-sectional profilethat is smaller than the inner cross-sectional area of the other and isable to seat inside another structural member, the members beingretained with a first end secured with a first vertebral body, and asecond end operatively fixed with a second vertebral body. Theorthopedic device may comprise at least two structural members, each ofwhich has a non-uniform longitudinal cross-sectional area.

Structural members may have the ability to be retained at anchoragepositions in any orientation along the transverse plane and,furthermore, have the ability to interface with one another inorientation along the transverse plane.

The orthopedic device may comprise at least two structural members whosegeometry allows the two to be mated together and received into eachanchorage point for securement at each level.

The orthopedic device may comprise a length appropriate helical coilspring with corresponding attachment fittings at each end. Eachattachment fitting may have the ability to be secured to each attachmentpoint. While securely attached to the helical coil spring, each fittinghas the ability to translate radially (or rotationally) with respect tothe anchorage point which effectively changes the geometric condition ofthe helical coil spring (reduce or enlarge the diameter). A lengthappropriate cylindrical rod may be located concentrically with thehelical coil spring.

An orthopedic device which may comprise at least one helical coil spring(compression) concentrically located inside an additional helical coilspring (extension) the outer helical coil spring anchored to eachvertebral body the inner helical coil spring retained to each anchoragepoint at each vertebral body. The anchorage points may interface witheach helical coil spring having longitudinal adjustability, andadditionally have the ability to receive a cylindrical rodconcentrically to both helical coil springs for another opportunity toalter the stiffness of the device.

An orthopedic device may comprise a pressure vessel which is placed inthe vicinity of and attached to each anchorage point, the pressurevessel having two or more independent, directional flow restrictingvalves. One valve may be for allowing fluid delivery into the pressurevessel, while another valve may serve to permit fluid exiting thepressure vessel. The valves may be disposed in a plurality ofconfigurations including being integral with the structural members,being disposed on an external line thereto, or being disposed with areservoir and system for adjusting the pressure/volume of the pressurevessel, any of such components (i.e., the valve, line, reservoir, andpressure system and actuator therefor) being disposed eithersubcutaneous or extracorporeal.

An orthopedic device may comprise a piston/cylinder configuration whichis oriented longitudinally and secured to each anchorage point on eachvertebral body, the piston having flow orifices of which the same couldbe adjusted to vary the volumetric flow rate and, ultimately, devicestiffness.

An orthopedic device may comprise a pressure vessel which is locatedlongitudinally between and attaches to each anchorage point, thepressure vessel additionally having an integrated reservoir which couldbe accessed post operatively for the purpose of introducing or removingworking fluid to/from the pressure vessel.

An orthopedic device may comprise a pressure vessel which is locatedlongitudinally between and attaches to each anchorage point, thepressure vessel having two independent, directional flow restrictingvalves. The first valve would allow a pressurized gas to be deliveredinside the pressure vessel. The second valve would allow pressurized gasto exit the pressure vessel.

In an aspect of the invention, a spinal stabilization system securablewith a plurality of vertebrae is disclosed including at least one anchorfor at each of least two vertebrae, and a spanning structure extendingbetween and securable with the anchors, wherein the spanning structurehas an adjustable mechanical performance characteristic.

In some forms, the mechanical performance characteristic is a bendingstiffness. The bending stiffness may be adjustable in orientationrelative to the vertebrae. The bending stiffness may be adjustable inanterior, posterior, lateral, and torsional modes. The bending stiffnessmay be selected by selection of cross-sectional areas of the spanningstructure. The bending stiffness may be selected by selection ofdiffering materials for the spanning structure.

In some forms, the spanning structure may include an outer member and aninner portion, wherein the bending stiffness may be selected byselection of the inner portion. The inner portion may be provided aftersecuring the outer member with the anchors. The inner portion may becomprised of a plurality of inner components, and the bending stiffnessmay be selected by selecting a number of the components to be disposedwithin the outer member. The bending stiffness may be adjusted byremoval or addition of the inner components. The bending stiffness maybe adjusted by orientation of the inner portion relative to the outermember. At least one of the outer member and the inner portion may haveeccentrically positioned regions of reduced cross-sectional area, androtation of the regions provides a direction for lowered stiffness.

In some forms, the mechanical performance characteristic is acompression/expansion stiffness. The spanning structure may include aspring including a plurality of coils. The stiffness may be adjustableby adjusting at least one physical characteristic of the spring. Thephysical characteristic may include at least one of the number of coils,the diameter of the coils, and the length of the spring. The coil springmay be an outer member, and the spanning structure may further includean inner portion, wherein the coil spring may provide a selectable andadjustable compression/expansion stiffness, and the inner portion mayprovide a bending stiffness.

In some forms, the spanning structure may includes a pair of springseach having a plurality of coils, wherein a first of the springs mayprovide a compression characteristic and a second of the springs mayprovide an expansion characteristic. The spanning structure may furtherinclude an inner portion, wherein one of the springs of the pair formsan outer spring, the other of the springs forms an inner spring, and theinner portion is disposed within the inner spring, the inner portionproviding a bending stiffness.

In some forms, the spanning structure may include a piston assemblycompressible and expandable along a longitudinal axis thereof. Thepiston assembly may be provided with compressible gas. The pistonassembly may be provided with substantially incompressible fluid. Thepiston assembly may be provided with a damper. In some forms, the pistonassembly is provided with fluid of mixed phases, a portion of the fluidbeing compressible gas and a portion of the fluid being incompressibleliquid. In some forms, the piston assembly is provided with fluid, andthe amount of fluid may be adjusted to adjust the mechanical performancecharacteristics. The system may further include a reservoir for fluid,wherein the piston assembly communicates with the reservoir, and themechanical performance characteristics may be adjusted by increasing thefluid in the piston assembly by delivering fluid thereto from thereservoir and may be adjusted by decreasing the fluid in the pistonassembly by delivering fluid therefrom to the reservoir. The pistonassembly and reservoir may be connected via at least two one-way valvesfor fluid transfer there between. The reservoir may be a compressiblebladder implanted subcutaneously.

In another aspect, a spinal stabilization system securable with aplurality of vertebrae is disclosed including at least one anchor for ateach of least two vertebrae, and a plurality of spanning structuresextending between and securable with the anchors, each spanningstructure having an adjustable mechanical performance characteristic.

In some forms, each of the spanning structures is adjusted to impart adifferent stiffness characteristic between its respective anchors. Insome forms, the mechanical performance characteristic of the spanningstructures may be adjusted after being secured with the anchors. In someforms, the mechanical performance characteristic for at least one of thespanning structures is a bending stiffness, and the mechanicalperformance characteristic for at least one of the spanning structuresis a compression/expansion stiffness.

In another aspect, a spinal stabilization system securable with aplurality of vertebrae is disclosed including at least one anchor for ateach of least two vertebrae, and spanning structures extending betweenand securable with the anchors, the spanning structure having anadjustable mechanical performance characteristic, wherein the mechanicalperformance characteristic is adjustable after the spanning structure issecured with its respective anchors.

In some forms, at least one spanning structure mechanical performancecharacteristic is adjustable via a percutaneous incision in a patient'sskin. In some forms, at least one spanning structure is adjustable viaan end thereof. In some forms, the system may be adjusted via animplanted key or tool without an incision. In some forms, at least onespanning structure mechanical performance characteristic is adjustablevia a hypodermic needle.

In some forms, at least one spanning structure includes a pistonassembly, and the system further including a reservoir for fluid,wherein the piston assembly communicates with the reservoir, themechanical performance characteristics of the piston assembly beingadjustable by increasing the fluid in the piston assembly by deliveringfluid thereto from the reservoir and adjustable by decreasing the fluidin the piston assembly by delivering fluid therefrom to the reservoir.The reservoir may be a compressible bladder implanted subcutaneously.

Incorporated by reference in their entireties are the following U.S.patents and patent applications directed generally to methods andapparatus related to surgical procedures, thus providing writtendescription support for various aspects of the present disclosure. TheU.S. patents and pending applications incorporated by reference are asfollows: U.S. Pat. Nos. 7,957,824, 7,844,356, 7,658,610, 6,830,570,6,368,325, 3,486,505 and U.S. Pat. Pub. Nos. 2010/0217336, 2009/0138020,2009/0087276, 2008/0161817, 2008/0114370, and 2007/0270875.

Additionally, U.S. Pat. Nos. 8,758,357, 8,870,889, 9,198,678 and9,642,633 are incorporated by reference for the express purpose ofillustrating systems and methods for creating a device, such as the onedescribed herein, using additive manufacturing or other techniques,wherein the device incorporates one or more patient-matched surfaces oris otherwise customized to a particular patient.

The phrases “at least one,” “one or more,” and “and/or,” as used herein,are open-ended expressions that are both conjunctive and disjunctive inoperation. For example, each of the expressions “at least one of A, Band C,” “at least one of A, B, or C,” “one or more of A, B, and C,” “oneor more of A, B, or C,” and “A, B, and/or C” means A alone, B alone, Calone, A and B together, A and C together, B and C together, or A, B andC together.

Unless otherwise indicated, all numbers expressing quantities,dimensions, conditions, and so forth used in the specification andclaims are to be understood as being approximations which may bemodified in all instances as required for a particular application ofthe novel apparatus described herein.

The term “a” or “an” entity, as used herein, refers to one or more ofthat entity. As such, the terms “a” (or “an”), “one or more” and “atleast one” can be used interchangeably herein.

The use of “including,” “comprising,” or “having” and variations thereofherein is meant to encompass the items listed thereafter and equivalentsthereof as well as additional items. Accordingly, the terms “including,”“comprising,” or “having” and variations thereof can be usedinterchangeably herein.

It shall be understood that the term “means” as used herein shall begiven its broadest possible interpretation in accordance with 35 U.S.C.,Section 112(f). Accordingly, a claim incorporating the term “means”shall cover all structures, materials, or acts set forth herein, and allof the equivalents thereof. Further, the structures, materials, or actsand the equivalents thereof shall include all those described in theSummary, Brief Description of the Drawings, Detailed Description,Abstract, and Claims themselves.

The Summary is neither intended, nor should it be construed, as beingrepresentative of the full extent and scope of the present disclosure.Moreover, references made herein to “the present disclosure” or aspectsthereof should be understood to mean certain embodiments of the presentdisclosure, and should not necessarily be construed as limiting allembodiments to a particular description. The present disclosure is setforth in various levels of detail in the Summary as well as in theattached drawings and the Detailed Description, and no limitation as tothe scope of the present disclosure is intended by either the inclusionor non-inclusion of elements or components when describing certainembodiments herein. Additional aspects of the present disclosure willbecome more readily apparent from the Detailed Description, particularlywhen taken together with the drawings.

The above-described benefits, embodiments, and/or characterizations arenot necessarily complete or exhaustive, and in particular, as to thepatentable subject matter disclosed herein. Other benefits, embodiments,and/or characterizations of the present disclosure are possibleutilizing, alone or in combination, as set forth above and/or describedin the accompanying figures and/or in the description herein below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutea part of the specification, illustrate embodiments of the disclosure,and together with the Summary and the Detailed Description serve toexplain the principles of these embodiments. In certain instances,details that are not necessary for an understanding of the disclosure orthat render other details difficult to perceive may have been omitted.It should be understood, of course, that the present disclosure is notnecessarily limited to the particular embodiments illustrated herein.Additionally, it should be understood that the drawings are notnecessarily to scale. In the drawings:

FIG. 1 shows a side elevation view of a fixation device according to oneembodiment of the present disclosure;

FIG. 2 shows a detailed view of the fixation device of FIG. 1;

FIG. 3 shows a sectional view of a fixation device according to anotherembodiment of the present disclosure;

FIG. 4 shows a detailed view of the fixation device of FIG. 3;

FIG. 5 shows a sectional view of a fixation device according to anotherembodiment of the present disclosure;

FIG. 6 shows an elevation view of a fixation device according to anotherembodiment of the present disclosure;

FIG. 7 shows a detailed view of the fixation device of FIG. 6;

FIG. 8 shows a sectional view of a fixation device according to anotherembodiment of the present disclosure; and

FIG. 9 shows a perspective, sectional view of the fixation device ofFIG. 8;

In the Figures, FIG. 10 is a perspective view of a first form of aspinal stabilization system secured with a plurality of representativeadjacent vertebrae, the stabilization including a plurality of anchorsin the form of pedicle screws and a plurality of spanning structuresconnecting the anchors, the spanning structures having a selectable andadjustable stiffness in bending or flexure provided by portions ofreduced cross-sectional area;

FIG. 11 is an exploded view of the stabilization system and vertebrae ofFIG. 10 showing the spanning structures having an outer shell portionand an inner core portion, the shell and core each having portions ofreduced cross-sectional area and being positionable relative to eachother and to the anchors to provide a desired stiffness in a directionor region for the stabilization system;

FIG. 12 is a top plan view of the stabilization system and vertebrae ofFIG. 10 showing the spanning structures received within channels ofyokes of the anchors;

FIG. 13 is a exploded view of the stabilization system and vertebraecorresponding to FIG. 12 showing the reduced cross-sectional areaportions of the cores having different orientations relative the shellreduced cross-sectional areas, as well as the anchors of thestabilization system, to provide different stiffness or mechanicalperformance characteristics to the different spanning structures;

FIG. 14 is a side elevational view of a pair of anchors secured with avertebra in cross-section, and of spanning structures of thestabilization system of FIG. 10 positioned for securement in the yokechannel thereof, an end of the spanning structure having structure forcooperating with a key or tool for adjusting the position of the corerelative to the shell;

FIG. 15 is a representative side elevational view showing an implantedstabilization system having a layer of flesh covering the stabilizationsystem, and access passages through the flesh provided by separateincisions, the access passages allowing access to end of spanningstructures of the stabilization system;

FIG. 16 is a representative view of a form of a stabilization systemhaving spanning structures formed of different materials to providedifferent moduli of elasticity thereto;

FIG. 17 is a perspective view of a form of a stabilization systemsecured with representative adjacent vertebrae, the stabilization systemincluding spanning members that are provided as multiple pieces joinedin the yoke of the anchor to provide different stiffness characteristicsbetween different vertebral levels;

FIG. 18 is a side elevational view of the stabilization system of FIG.17 showing spanning structures of an upper vertebral level having agreater cross-sectional thickness than spanning structures of a lowervertebral level;

FIG. 19 is a partially exploded view of the stabilization system of FIG.17 showing a portion of the spanning structure of the upper vertebrallevel removed, and showing unitary structures disposed in yokes for boththe upper and lower vertebral levels;

FIG. 20 is a top plan view of a form of a stabilization system securedwith representative adjacent vertebrae, the stabilization system havingspanning structures including spring coil portions securable with thechannels of the yokes and having end fixtures that are graspable ormanipulable with a tool for rotating the end fixtures to alter thestiffness characteristics of the spanning structures;

FIG. 21 is an exploded perspective view of a form of the stabilizationsystem of FIG. 20 showing rod-like central core portions receivablewithin the coil portions of the spanning structure;

FIG. 22 is a side elevational view of a form of a spanning structure foruse with anchors, the spanning structure having a outer sheath or casingwhich permits addition or removal of core strands there within forproviding a selected stiffness to the spanning structure;

FIG. 23 is a perspective view of a form of a stabilization systemsecured with representative adjacent vertebrae, the stabilization systemhaving anchors with posts for engaging with spanning structures havingcoil springs with end loops;

FIG. 24 is a side elevational view of the stabilization system of FIG.23;

FIG. 25 is an exploded perspective view of a form of the stabilizationsystem of FIG. 23 showing the coil springs as outer coil springs,showing inner coil springs, and showing central rod-like core membersfor providing desired stiffness characteristics to the spanningstructures;

FIG. 26 is an exploded view of an anchor of FIG. 23 showing a nut forsecuring the post within a recess of the anchor base, a bore in the postfor receiving a core member, and a groove in the post for receiving anend loop of an outer coil spring;

FIG. 27 is a perspective view of a stabilization system secured withrepresentative adjacent vertebrae, the stabilization system includingspanning structures having piston assemblies selectively pressurizedwith fluid such as gas;

FIG. 28 is a top plan view of the stabilization system of FIG. 27;

FIG. 29 is a perspective view of a stabilization system secured withrepresentative adjacent vertebrae, the stabilization system includingspanning structures having piston assemblies selectively filled withfluid such as liquid;

FIG. 30 is a top plan view of the stabilization system of FIG. 29; and

FIGS. 31A-31C are cross-sectional views of spanning structures for usein stabilization systems having varying spring and stiffnesscharacteristics along their length.

Similar components and/or features may have the same reference number.Components of the same type may be distinguished by a letter followingthe reference number. If only the reference number is used, thedescription is applicable to any one of the similar components havingthe same reference number.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure has significant benefits across a broad spectrumof endeavors. It is the Applicant's intent that this specification andthe claims appended hereto be accorded a breadth in keeping with thescope and spirit of the disclosure and various embodiments disclosed,despite what might appear to be limiting language imposed by specificexamples disclosed in the specifications. To acquaint persons skilled inthe pertinent arts most closely related to the present disclosure,preferred and/or exemplary embodiments are described in detail withoutattempting to describe all of the various forms and modifications inwhich the novel apparatus, devices, systems and methods might beembodied. As such, the embodiments described herein are illustrative,and as will become apparent to those skilled in the arts, may bemodified in numerous ways within the spirit of the disclosure.

By way of providing additional background, context, and to furthersatisfy the written description requirements of 35 U.S.C. § 112, thefollowing are incorporated by reference in their entireties for theexpress purpose of explaining and further describing the various toolsand other apparatus commonly associated therewith surgical procedures,including minimally invasive surgery (“MIS”) procedures: U.S. Pat. No.6,309,395 to Smith et al.; U.S. Pat. No. 6,142,998 to Smith et al.; U.S.Pat. No. 7,014,640 to Kemppanien et al.; U.S. Pat. No. 7,406,775 toFunk, et al.; U.S. Pat. No. 7,387,643 to Michelson; U.S. Pat. No.7,341,590 to Ferree; U.S. Pat. No. 7,288,093 to Michelson; U.S. Pat. No.7,207,992 to Ritland; U.S. Pat. No. 7,077,864 Byrd III, et al.; U.S.Pat. No. 7,025,769 to Ferree; U.S. Pat. No. 6,719,795 to Cornwall, etal.; U.S. Pat. No. 6,364,880 to Michelson; U.S. Pat. No. 6,328,738 toSuddaby; U.S. Pat. No. 6,290,724 to Marino; U.S. Pat. No. 6,113,602 toSand; U.S. Pat. No. 6,030,401 to Marino; U.S. Pat. No. 5,865,846 toBryan, et al.; U.S. Pat. No. 5,569,246 to Ojima, et al.; U.S. Pat. No.5,527,312 to Ray; and U.S. Pat. Appl. No. 2008/0255564 to Michelson.

Referring now to FIGS. 1-9, varying embodiments of the presentdisclosure are shown. The fixation devices shown in FIGS. 1-9 preferablycomprise a porous or fenestrated structure, which may be exposed atspecific regions along the fixation device, and which aid withosteo-integration while increasing the mechanical stability of thedevice. Various benefits of the fixation device, which in a preferredembodiment is in the form of a screw, are described herein.

In the embodiment of FIG. 1, the fixation device is provided in the formof a screw having a head portion 10 and a distal portion 20, withthreads 30 located on the shaft 15 between the head portion 10 and thedistal portion 20. In certain embodiments, the head portion 10 isaffixed to the body of the fixation device, while in other embodimentsthe head is allowed to float and thereby provide a poly-axial screwassembly. In other embodiments, the head may be temporarily attached tothe body of the fixation device. According to this particularembodiment, the porous or fenestrated structure is comprised of theshaft 15 of the screw, but not covering the head portion 10 or thethreads 30. The number of fenestrations or porosity of the porousstructure of the screw may be greater or lesser than depicted in FIG. 1.

Referring now to FIG. 2, a detailed view of the porous screw isdepicted. In this drawing, the fenestrations 40 are arranged in seriesand extend substantially the entire width of the shaft 15 between thethreads 30. In alternate embodiments, the fenestrations may be fewer innumber than shown, and may not be uniform in their location andarrangement. The fenestrations may be circular or any other shapesuitable for the fixation device. In alternate embodiments, thefenestrations extend even through the threads 30 of the screw.

The screw may be additively manufactured such that the porous structuremay be exposed to the interfacing bone but also contained within thecore of the screw. The screw may be additively manufactured, by way ofexample but not limitation, out of biocompatible alloys, including byusing electron-beam melting or selective laser sintering methods toproduce various surface finishes on the porous and solid aspects of thescrew. The screw may be a manufactured as a single part, fixed anglescrew, or may be poly-axial.

Referring now to FIGS. 3-4, the screw may comprise a substantiallyhollow section that is in communication with the plurality offenestrations 40. For example, as shown in FIG. 3, the screw maycomprise a porous structure in a solid core to provide superiormechanical characteristics. The porous structure may be confined to onlya certain region of the screw, or may extend substantially the entirelength of the screw.

Referring now to FIG. 5, the fenestrations 40 may extend the entirewidth of the screw for substantially the entire length of the shaft 15,as shown. In this manner, the number of fenestrations 40 may beincreased, thereby increasing the number of locations along the outersurface of the screw that either material can be introduced (via thescrew) or material may be collected (via suction as described above). Inembodiments, the screw may have a longitudinal channel 35 that runslengthwise through the screw and permits material to be introducedthrough the longitudinal channel and into the plurality of fenestrations40. For example, the longitudinal channel may be accessible from thehead portion 10 of the screw, such that an operator may inject fluid orsolid material into the longitudinal channel 35 and ultimately throughthe porous structure of the screw via fenestrations 40. In a similarmanner, the fenestrations 40 (which surround the outer surface of theshaft 15 of the screw) may be used with vacuum or suction applied to thehead portion 10 of the screw to retract material adjacent to thefenestrations 40 and either retained by the porous structure of thescrew or within the longitudinal channel 35. By way of example, thescrew could either be infused with bone morphogenetic protein orequivalent to enhance fusion between the screw and the patient'sanatomy, or the screw could suction and retain autogenous blood into thefenestrations, which in turn stimulates boney ingrowth in the poroussection of the screw. Variations on this embodiment are consideredwithin the scope of the present disclosure.

Referring to FIG. 6, one embodiment comprises a varying array offenestrations 40, which may increase or decrease along the length of thescrew. As shown in FIG. 6, the number of fenestrations 40 may vary in asecond portion of the screw, such that there is a decreased number orpattern of fenestrations 40′ than at a different portion of the screw.In other portions of the screw, the shaft 15 may not comprise anyfenestrations at all. This variation is best shown in the detailed viewof FIG. 7. In this manner, material may either be collected orintroduced via the screw only at desired locations along the length ofthe screw, which may be desirable given the particular bone density andsurrounding anatomical features of a particular patient. In reference toFIGS. 6 and 7, in certain embodiments the longitudinal channel maycomprise additional fenestrations in certain portions to betteraccommodate a particular patient's anatomy. For instance, a firstportion of the longitudinal channel 35 is as depicted in FIG. 3, while asecond portion of the longitudinal channel 35′ may comprise a greaternumber of fenestrations and therefore a greater porosity.

Further variations of the embodiments described above are shown in FIGS.8-9. For example, the fixation device may be a pedicle screw, which maybe additively manufactured such that the solid and porous aspects of thescrew are fused together into a single solid construct. In otherembodiments, the solid and porous structure may be co-extensive with theporous elements interdigitated within and around various solid elementsof the screw. Variations on these embodiments are considered within thespirit of the presently claimed invention.

The porous elements of the fixation device, screw or implant may bedesigned to more closely resemble that of a specific patient's anatomy.Accordingly, one advantage of the present disclosure is to promote bonyingrowth throughout the porous portions or sections of the implant,which in turn reduces the risk of loosening of the device under dynamicloading situations.

In still other embodiments, only a portion of the screw is manufacturedwith a porous surface. For example, the exposed porous aspects of thescrew may be localized along the minor diameter of the thread form. Thescrew may therefore comprise hollow, porous or solid core elements toallow for varying levels of implant stiffness. These areas may besurrounded by a mostly solid thread form to facilitate smoothimplantation and firm seating of the screw. Particular reference is madeto FIGS. 3-4 when referring to this embodiment.

As referred to above, the porous elements of the screw may be designedwith localized fenestrations, which may be at least partially accessiblefrom the screw head, and thereby facilitate delivery of osteogenicagents such as bone-morphogenetic proteins, HA and or allograph orautograph tissue into the porous portions of the screw. This in turnallows for bony ingrowth and greater pullout resistance, as describedabove. The exposed porous structure may be located on the proximalportion of the screw, adjacent the screw head, in order to localizeingrowth. Localization of ingrowth may increase mechanicalcharacteristics of the bone screw interface, and subsequently allow foreasier implant removal in the case of revision surgery. The localizedporous elements may be tapered outward to increase the interference fitof the porous elements with the surrounding anatomy.

The porous features are representative of porous cancellous bone withporosity preferably ranging between 30-80% to allow for ingrowth ofosteocytes and supporting vasculature. Stated another way, the porousfeatures, when compared to the solid features of the device, make upabout 30-80% of the volume of the device. In a most preferredembodiment, the porosity is about 50%. In certain embodiments, theporous structure may be regular and geometric or irregular in form. Inyet other embodiments, the porous density may be homogenous throughoutthe screw, or may be heterogeneous in order to attain desired stiffnessand or improve the structural interface of the solid and porouselements.

The implant length and diameter may be pre-surgically planned to matchthe anatomical size of the patient's anatomy. The implant porosity andsubsequent modulus may be pre-surgically planned to match the bonedensity of the intended patient. For example, in one embodiment, thesurgical devices described above may be matched to an anatomic featureof a patient that has degenerated and needs to be restored. In anotherembodiment, the surgical device may be necessary to correct structuralor physiological deformities present in the patient anatomy, and therebyserve to correct position or alignment of the patient anatomy. Otherdevices may be patient specific but do not serve a restorative or“structural” function.

The surgical devices described herein may be manufactured via additivemanufacturing. In the context of spinal implants, the surgical devicesmay be used in all approaches (anterior, direct lateral, transforaminal,posterior, posterior lateral, direct lateral posterior, etc). Specificfeatures of the surgical device can address certain surgical objectives,for example restoring lordosis, restoring disc height, restoringsagittal or coronal balance, etc. The fixation and surgical devicesdescribed herein may then be fabricated by any method. Fabricationmethods may comprise the use of a rapid prototyping machine, a 3Dprinting machine, a stereolithography (STL) machine, selective lasersintering (SLS) machine, or a fused deposition modeling (FDM) machine,direct metal laser sintering (DMLS), electron beam melting (EBM)machine, or other additive manufacturing machine.

To add further stability to the seating and placement of the fixationdevices described herein to the patient anatomy, the outer surfaces ofthe fixation device may further comprise one or more spikes or teeth orother surface features, which serve to contact and at least partiallypenetrate or “grip” the patient anatomy to secure the fixation device inplace. In one embodiment, the surface features may be made of the samematerial and may be permanently attached to the fixation device. Inanother embodiment, the surface features may be comprised of an overlay,and/or may be made of a different material, such as the ones describedherein, and may further be selectively inserted onto the fixationdevice(s) as desired.

One having skill in the art will appreciate that embodiments of thepresent disclosure may have various sizes. The sizes of the variouselements of embodiments of the present disclosure may be sized based onvarious factors including, for example, the anatomy of the patient, theperson or other device operating with or otherwise using the apparatus,the surgical site location, physical features of the devices andinstruments used with the apparatus described herein, including, forexample, width, length and thickness, and the size of the surgicalapparatus.

One having skill in the art will appreciate that embodiments of thepresent disclosure may be constructed of materials known to provide, orpredictably manufactured to provide the various aspects of the presentdisclosure. These materials may include, for example, stainless steel,titanium alloy, aluminum alloy, chromium alloy, and other metals ormetal alloys. These materials may also include, for example, PEEK,carbon fiber, ABS plastic, polyurethane, polyethylene, photo-polymers,resins, particularly fiber-encased resinous materials, rubber, latex,synthetic rubber, synthetic materials, polymers, and natural materials.

One having skill in the art will appreciate that embodiments of thepresent disclosure may be used in conjunction devices that employautomated or semi-automated manipulation. Various apparatus and implantsdescribed herein may be provided to facilitate or control the entrypoint, angular trajectory, height, and/or head orientation of a screw,for example. This is desirable, particularly with placement of screws inthe human body, as it permits a surgeon/user to optimize spinal screwhead alignment for subsequent rod insertion across multiple boneylandmarks. Additionally, by controlling screw placement, a patientspecific rod may be designed and manufactured to either match thepre-planned screw placement, or offer angular corrections in order tooptimize curvature of the spine.

The present disclosure may also be advantageous in light of recentimprovements in decentralized manufacturing. For example, surgicaldevices may soon be capable of fabrication in a number of different andconvenient settings, including but not limited to an off-sitemanufacturing location, an on-site manufacturing location, usingequipment present in a surgeon's clinic or offices or in a public orprivate hospital. For example, modules may be fabricated based on aparticular patient need and immediately fabricated once the need isidentified, and then provided directly to the surgeon.

In accordance with another aspect of the present disclosure, a pluralityof forms and embodiments of spinal stabilization systems are depicted inFIGS. 10-31. In a variety of manners, these forms provide a user-surgeonwith a range of choices for the motion that is permitted for spanningstructures of the spinal stabilization system, the mechanical propertiesof the spanning structures including flexure, torsion, and/orcompression and expansion, with linearly selectable mechanicalproperties, provide a surgeon with spanning structures that can providea range of mechanical properties while being used with identical yokesof anchors, allow the surgeon to adjust the mechanical properties insitu, and allow the surgeon to adjust the mechanical propertiespost-operative without full-scale surgical revision.

Referring to FIGS. 10-14, a first form of a spinal stabilization system10 of the present invention is illustrated secured with a plurality ofrepresentative vertebrae V. As illustrated, the vertebrae V include aninferior vertebra VI, a medial vertebra VM, and a superior vertebra VS.The stabilization system 10 includes a plurality of anchors 12 so that apair of anchors 12 is provided for each vertebra V, as is well-known inthe art. Each anchor 12 includes a screw 14 having a threaded shank 16received in its respective vertebra V and includes a yoke 18. In someforms, the yoke 18 and shank 16 may be fixed relative to each other,such as by the anchor 12 being a unitary component or by being formingintegral. In other forms, the anchor 12 may be a poly-axial anchor sothat the yoke 18 may be oriented in a desirable manner once the anchorshank 16 is secured with the vertebra V.

The stabilization system 10 includes spanning structures 20 forconnecting the vertebra V to control the relative movement therebetween. Each yoke 18 includes a channel 22 into which one or morespanning structures 20 is received for securement therewith. Once aspanning structure 20 is properly seated in the channel 22, a securement(not shown) generally referred to as a cap is driven atop the spanningstructure 20 such as by being threaded into arcuate recesses 24 of theyoke 18 and to the sides of the channel 22.

As best seen in FIGS. 11 and 13, each spanning structure 20 is generallyrod-like with an outer surface 30 with a plurality of cut-outs orscallops 32. The scallops 32 provide stress concentrators or,alternatively viewed, regions of lower stiffness for the spanningstructure 20. When the spanning structure 20 is secured with the yokes18, the scallops 32 are oriented in a direction in which it is desiredto permit greater flexure between the anchors 12 to which the spanningstructure 20 extends. To be clear, the scallops 32 are areas of reducedcross-sectional area that are eccentrically positioned relative to thecentral longitudinal axis of the spanning structure 20 so thatorientation of the spanning structure 20 provides a distinct directionof lowered stiffness, and so that rotation of the spanning structure 20alters the direction of lowered stiffness.

As can be seen, a first spanning structure 20 a is secured between afirst yoke 181 secured with the inferior vertebra VI and with a secondyoke 18M secured with the medial vertebra VM while a second spanningstructure 20 b is secured between the second yoke 18M and a third yoke18S secured with the superior vertebra VS. When secured, the scallops 32of the first and second spanning structures 20 a, 20 b may havedifferent radial orientations such that the flexure mechanicalcharacteristics between the first and second yokes 18I and 18M aredifferent than the flexure mechanical characteristics between the secondand third yokes 18M and 18S.

It should also be recognized that the first spanning structure 20 acooperates with a third spanning structure 20 c while the secondspanning, structure 20 b cooperates with a fourth spanning structure 20b to define the mechanical properties between their respective vertebraeV; thus, varying the orientations of scallops 32 for each of the fourspanning structures 20 a-20 b serve to provide at least some of themechanical properties for the stabilization system 10 as a whole. Itshould also be noted that the materials of the different spanningstructures 20 a-20 b may be varied to provide or influence themechanical properties of each.

In a further form of the spanning structure 20, the scallops 32 areformed on a shell member 40, and a core member 42 is received within theshell 40. In various forms, the core 42 may be of like or dissimilarmaterials to influence the mechanical properties to provide varyingselected or selectable flexure properties, for instance.

In a preferred form, the core 42 also includes scallops 44 along itslength, as best seen in FIGS. 11 and 13. When the core 42 is receivedwithin the shell 40, the core scallops 44 may be aligned (or misaligned)to varying degrees with the shell scallops 32. As should be evident,when the sets of scallops 44, 32 are aligned, such augments the flexurecharacteristics and, more appropriately, lessens the stiffness of thespanning structure 20 as a whole in a particular direction. When thescallops 44, 32 are largely misaligned, the decrease in stiffnessprovided by the different scallops 44, 32 is aligned in first and seconddirections. For the scallops 44, 32 merely being partially overlappingor relatively juxtaposed, the decrease in stiffness is distributed overthe region between and including the scallops 44, 32. It should be notedthat the scallops 32, 44 may be aligned or misaligned in both a radialdirection (i.e., orientation in a 360 degree sweep) and in an axialdirection.

The alignment of the scallops 32, 44 may be selected at any time priorto, during, or after implantation (securement in the yokes 18), as wellas after the surgical procedure itself. To promote such adjustment, thecore 42 may be provided with structure 50 on one or more ends 52 forengaging and rotating the core 42 relative to the shell 40.

As can be seen in FIG. 14, the core 42 includes a socket 56 shaped forreceiving a key 58 (not shown). As an example, the socket 56 may behexagonal (FIG. 14) for receiving a hexagonal key 58 (FIG. 15). In otherforms, the key 58 may have a hook (not shown) or the like for axiallyadvancing or withdrawing the core 42 along the axial direction of theshell 40. In another form, the socket 56 may include a section ofinternal threading for threadably receiving the key 58, the key 58having slightly undersized threading (FIG. 10) for easy thread-receiptand effecting rotation in a single direction when fully advanced in thesocket 56. Due to the threaded connection, such key 58 enables axialforces to be applied to the core 42 to advance/withdraw the core 42within the shell 40.

The scallops 32, 44 may be cut at an oblique angle relative to acircumference of the shell 40 and/or core 42 so that the scallops 32, 44may also facilitate or enable torsional distortion thereof. The depth,frequency, and/or size of the scallops 32,44 may be varied along thelength of the shell 40 or core 42 so that the “spring equation” of thespanning structure 20 is non-linear, that is, so that the force requiredto achieve a certain amount of bending to the spanning structure 20increases as the bending increases. Instead of the scallops 32, 44,either or both of the shell 40 and/or core 42 may simply be given anon-circular cross-section so that the bending characteristics are notthe same throughout a 360 degree sweep.

Turning now to FIG. 15, the spinal stabilization system 101 is depictedas implanted with a layer 60 of a patient's flesh (including the surfaceskin) located atop the stabilization system 10. As can be seen, a smallincision 62 may be made in the layer 60 to provide a passage or access64 to the end 52 of a spanning structure 20. The key 58 may be insertedthrough the small incision 62 and the access 64 for connection with thespanning structure 20 socket 56. Accordingly, a major revision surgicalprocedure is not necessary to alter the mechanical performancecharacteristics (i.e., flexure or stiffness of the spanning structures20), as such can be done with a minor procedure. It should also be notedthat the core 42 may be entirely removed from the shell 40, which wouldalso permit a new core 42 with greater or lesser stiffness to replacethe previous core, all without having to remove the securements (i.e.,caps) from the yokes 18.

As discussed above, the materials for the spanning structures 20 may bevaried to provide different flexure or mechanical performancecharacteristics. Turning to FIG. 7, a form of a spinal stabilizationsystem 80 is depicted similar to that of FIGS. 10-15, though simplifiedto illustrate spanning structures 82 and, in particular, to depict afirst spanning structure 82 a having a first modulus of elasticity and asecond spanning structure 82 b having a second modulus of elasticitythat is different from the first, the modulus of elasticity determinedby the material from which each spanning structure 82 a, 82 b is formed.As noted above, in the event a pair of spanning structures 82 is used intandem to span between two vertebrae V, such as adjacent vertebrae V,the flexure characteristics are determined by a combination of theelastic moduli of the two spanning structures 82 of the pair.

It should be noted that reference to flexure characteristics andmechanical performance characteristics, as used herein, are meant torefer to how a spanning structure and/or a stabilization system performsunder load, based on inherent materials properties and structuralgeometry. While in biomechanics, flexure and extension are generallythought of as being opposite, with respect to curving or bending of aspanning structure, these terms are one and the same. Additionally,these terms are intended in a broad manner to also include torsionaldistortion or twisting. Modulus of elasticity or elastic modulus is aninherent property of the material, regardless of shape or geometry.While stiffness and modulus of elasticity are typically thought of aslinear descriptions of mechanical behavior dependent on shape andmaterial, thereby equating them to a spring equation having a springconstant K (i.e., Force=K×Change in Length), it should be noted thatthese terms herein encompass a non-linear description of mechanicalbehavior such that force and distortion are not in direct proportion.

Turning now to FIGS. 17-19, a further form of a spinal stabilizationsystem 100 is illustrated having spanning structures 102 with differentand selectable flexure characteristics. Again, the stabilization system100 is largely similar to the stabilization systems 10 and 80, discussedabove. However, the flexure characteristics of the stabilization system100 of FIG. 17 are principally determined by the cross-sectional size ofthe spanning structures 102 as a whole between the vertebrae V.

More particularly, a spanning structure 102 a between the superior andmedial vertebrae VS and VM is approximately twice the cross-sectionalsize of the spanning structure 102 b between the medial and inferiorvertebrae VM and VI. As best seen in FIG. 19, the spanning structure 102a, 102 b both include portions of a base spanning structure 104 thatextends across and between each of the vertebrae V. However, thesuperior-medial spanning structure 102 a additionally includes asecondary spanning structure 106, the combination of the same with thebase spanning structure 104 defining the flexure characteristicstherefor. Accordingly, the stiffness of the superior-medial spanningstructure 102 a is greater than the stiffness of the medial-inferiorspanning structure 102 b.

To the degree each of the spanning structures discussed herein does notexceed its elastic limit (or, more precisely, its change in shape doesnot exceed, for any portion thereof, a change beyond which deformationbecomes permanent), such spanning structures may be modeled as a spring.However, each of the above-discussed forms of the spanning structuresprovides little, if any, expansion or compression along the longitudinalaxial direction of the spanning structures.

Turning now to FIGS. 20 and 21, a further form of a spinal stabilizationsystem 120 is shown having spanning structures 122 that include a coilspring portion 124 that allows the stabilization system 120 toaccommodate expansion and contraction of the spanning structure 122along its longitudinal axis. The stabilization system 120 includesanchors 12 and yokes 18, like each of the above-described embodiments,the spanning structures 122 being received in the yokes 18 and securedtherein by a securement such as a cap.

In order to secure the spanning structure 122 with the yokes 18, eachend 126 thereof includes an end fixture 128. The end fixture 128 mayhave any shape, provided that the end fixture 128 is generallysufficiently rigid as to be compressed within the yoke 18 by thesecurement. The end fixtures 128 are illustrated as being generallyoctagonal so that flats 130 are formed on the end fixture 128, a pair ofthe flats 130 contacting the sides of the yoke channel 22, a flat 130contacting the bottom interior of the yoke channel 22, and a flat 130being outwardly facing for contact with the cap when secured in the yoke18. As noted, other configurations of the end fixture 130 may beprovided, such as a square or circle; however, the octagonal shape hasthe benefit of a leading flat 130 that is shorter than the width of theyoke channel 22 to assist in initial advancement of the end fixture 128into the channel 22. The octagonal shape also provides the benefit ofthe flats 130 themselves for engaging with the yoke 18 and cap, whichserves to provide good compressive contact and serves to retard rotationof the end fixture 128 within the yoke 18 after securement.

Each spanning structure 122 is provided with a single coil spring 124.For the various spanning structures 122 illustrated, each can beprovided with varying mechanical performance characteristics. Forinstance, the effective (i.e., when implanted) spring constant for eachcoil spring 124 can be selected based on the length of the coil spring124, a number of turns in the coil spring 124, a diametral size of thecoil spring 124, and pre-stressing of the coil spring 124 whenimplanted.

A surgeon can easily adjust or alter the performance characteristics byaltering the above aspects of the coil spring 124. As best seen in FIG.20, each end fixture 128 is provided with at least one opening 136. Atool (not shown) can be inserted into the end fixture 128 through an endpassage 138 and into the opening 136. The tool can then be used torotate the end fixture 128 relative to the other end fixture 128, thuspres-stressing the coil spring 124 as well as changing the diametralsize and number of coils in the spring 124. In one form, a first of theend fixtures 128 may be positioned in a yoke 18, while the other ismanipulated as described. Alternatively or in addition, the first endfixture 128 may be secured in a yoke 18, and the other end fixture 128may be pulled longitudinally, along the axis of the spanning structure120, to remove it from the yoke 18; the end fixture 128 may then berotated and returned within its yoke 18 when the desired number of turnshas been made. In order to perform such, loosening of a cap orsecurement for the end fixture 128 that is rotated may be necessary,particularly if such procedure is performed in a post-operativeprocedure.

While the spanning structures 122 including the coil springs 124 provideexpansion and compression along the longitudinal length, they provideless stiffness in the other directions. Accordingly, a core 132 may beinserted within the coil springs 124. The cores 132 may be provided withvarying mechanical performance characteristics, as has been discussedherein, such as by being formed of materials with different elasticmoduli.

As shown in FIG. 21, the core 132 may span a plurality of vertebrae V.Alternatively, the cores 132 may span only to two adjacent vertebrae V.In a preferred form, the cores 132 may be removable and replaceablewithout removal of the securement and end fixtures 128. In this manner,the cores 132 may be changed by the above-described simple incisionprocedure. Towards this end, the cores 132 may be provided withstructure assisting in their removal, such as structure similar to theabove-described socket 56 and key 58.

In a form similar to the spanning structures 20 or 122, a stabilizationsystem may be provided with spanning structures 142 that are essentiallytubular casings 144, having a hollow bore 146, and a plurality ofstrands 148 of material are received within the bore 146, as depicted inFIG. 22. The number and/or size of the strands 148 thus cooperate withthe casing or sheath 144 to provide the flexure characteristics for thespanning structure 142. In general, the strands 148 would generally berod or wire-like with a constant diameter and inserted within the casing144 to provide a desired stiffness. However, the individual strands 148may also have non-uniform cross-sections, for the reasons discussedherein, and/or may have non-uniform lengths. For the latter, the strands148 could be staggered or otherwise positioned relative to each other sothat the combination of the strands 148 and the casing 144, through anyparticular cross-section, determine the stiffness thereat.

The number or configuration of the strands 148 may be modified at anydesired time, such as post-implantation or post-operatively. That is, itmay be convenient to initially implant and secure the casing 144 withthe yokes 18, and then insert the strands 148. Furthermore, later minorsurgical procedures could be performed to provide additional strands148, or to remove strands 148, based on the conditions experienced bythe patient.

It is known that the bone-screw interface, such as for a pedicle screw,improves over time in the absence (or minimization) of loading on theinterface. Therefore, it may be desirable for a portion of thestabilization systems to be implanted with minimal loading on theanchors 12, and a portion to be subsequently adjusted or added toincrease the loading on the anchors 12 or the stiffness of thestabilization system.

For instance, the casing 144 may be implanted (or the above-describedshell 40 or coil spring 124, for instance) with the bore 146substantially empty. After a period of time; a minor surgical procedureincluding a small incision proximate the spanning structure, as isdescribed for FIG. 15, may be performed to increase the stiffness suchas by inserting strands 148 into the bore 146.

In a reverse manner, decreasing the stiffness of the spanning structuresmay be performed in accordance with that discussed for FIG. 15 by makingthe small incision and removing strands 148 from the bore 146.

In another form of spinal stabilization system 160, shown in FIGS.23-26, anchors 162 are provided for securing spanning structures 164having springs. The anchors 162 include a threaded shank 166 asdescribed above and a head 168 which may or may not be polyaxiallyadjustable, as described. In contrast to the above forms, the head 168does not form a yoke 18 having a channel 22, instead having acylindrical recess 170 defined by an upstanding collar 172.

An anchor post 174 cooperates with the head 168 for securing thespanning structures 164 with the anchors 162. The post 174 includes awidened base 176 received in the recess 170 and an upstanding postportion 178. The head collar 172 is threaded (either internally orexternally) for receiving a nut 180 thereon for securing the anchor post174 with the head 168.

As best seen in FIG. 26, the post portion 178 includes a hollow or abore 184 into which a portion 190 of the spanning structure 164 isreceived. Specifically, the portion 190 is a rod-like member linearlyadvanced through a bore 184 of a first anchor 162 a and into a bore 184of a second anchor 162 b, representatively noted in FIG. 23. The postportion 178 receives a set screw 179 that may be driven into the postportion 178 to reach the bore 184 and apply pressure against the portionspanning structure rod 190.

The spanning structures 164 each include a first spring 194 and a secondspring 196 located, sheath-like, around the rod portion 190. The firstspring 194 has a smaller diameter than the second spring 196 so that thesecond spring 196 is also positioned, sheath-like, around the firstspring 194. The first spring 194 is configured to be compressed from anatural position when the stabilization system 160 is loaded so that theanchors 162 between which the first spring 194 spans are moved towardeach other. The second spring 196 is configured to be stretched orexpanded from a natural position when the stabilization system 160 isloaded so that the anchors 162 are moved away from each other. In orderto maintain the second spring 196 with the anchors 162, an end 198 ofeach second spring 196 includes an end loop 200 that may be securedaround the post portion 178 and, in particular, in an annular groove(not shown) formed in the post portion 178.

As described above, one manner of selectively varying the stiffness ofthe second (expansion) spring 196 coil is by rotation of the ends 198 toenlarger or contract the diameter of the spring 196, thereby changingits spring equation. It should be noted that the size of the coils maybe varied over the length of the spring 196 to give the springnon-linear spring/flexure characteristics. Similarly, the springproperties of the first (compression) spring 194 may be altered.

It should also be noted that the stabilization system 160 may also beadjusted through a small incision formed proximate an anchor 162 in amanner similar to that described for other forms herein. Removal of therod portion 190 and release of one of the ends 198 of the second spring196 allows the first spring 194 to be removed and changed, for instance,and the ends 198 may also be subsequently rotated and replaced on thepost portion 178.

Turning now to FIGS. 27-30, forms of spinal stabilization systems areshown using fluid and piston assemblies, fluid referring to both gassesand liquids. As will be discussed in greater detail below, a first formof such systems is shown in FIGS. 27 and 28 as stabilization system 220having a plurality of anchors 12 and spanning structures 222, eachhaving a gas-filled piston 224 assembly thereon. As will also bediscussed below, FIGS. 29 and 30 depict a stabilization system 250having a plurality of anchors 12 and spanning structures 252, eachhaving a liquid filled piston assembly 254 thereon.

Turning to FIGS. 27 and 28, the piston assembly 224 may be referred toas a pneumatic assembly including a fluid chamber (not shown) and apiston head (not shown) reciprocable within the chamber. The fluidchamber is filled with gas so that movement of the piston head therewithin serves to either compress or expand the gas within the chamber.Accordingly, to some degree, the gas acts as a spring.

The “stiffness” of the gas acting like a spring can be modified by asurgeon user. In a preferred form, an end 226 of each piston assembly224 includes a port 228 for connection with an external fluid reservoir(not shown) that allows a surgeon to pump in additional fluid or gas, orallows the surgeon to bleed off a portion of the gas. As in otherembodiments discussed herein, such pressure adjustment may be performedpost-operatively, such as through a small incision or via a hypodermicneedle injection. Additionally, a reservoir may be implantedsubcutaneously that allows for manual pumping of the reservoir, throughthe skin, and pressure relief. For instance, the reservoir may be acompressible bladder-type device connected via a one-way valve to injectfluid into the piston chamber, and a second one-way valve may beprovided for reducing or bleeding fluid from the piston chamber into thebladder.

The stabilization system 220 may be implanted with little or no gas sothat the bone-anchor interface is able to heal prior to loading of thestabilization system 220, as has also been discussed above, andsubsequently the piston assembly 224 may be pressurized as desired. Ascan be seen, different piston assemblies 224 of the stabilization system220 may be provided with different internal pressures within the pistonchamber so that each piston assembly 224 has a selected “stiffness.”

The stabilization system 250 of FIGS. 29 and 30 is similar in operationto that of FIGS. 27 and 28. The stabilization system 250 is a hydraulicsystem utilizing fluid in the form of a liquid that is incompressible orminimally compressible within piston assemblies 254. Accordingly, thepiston assembly 254 is highly resistant to compression or expansion.While this may be viewed as a detriment, it is noted that pumping in orbleeding off of liquid from a port 255 located on an end 256 of thepiston assembly 254 provides a high degree of predictability for theperformance of the piston assembly 254. In increasing or decreasing theliquid volume, the distance between the anchors 12 to which the pistonassembly 254 is secured is relatively easily determined by the surgeon;for instance, a surgeon may be using the stabilization system 250 torelieve pressure on a damaged intervertebral disc that is causingpressure and pain on the spinal column, and shifting of vertebrae awayfrom each other by increasing the liquid volume in the piston assembly254 is evident.

In a variation of the stabilization system 250, the piston assembly 254may be provided with a dashpot damping structure (not shown) within thefluid (or, more appropriately within the liquid-filled fluid chamber ofthe piston assembly 254). In this manner, controlled and moderatecompression or expansion of the piston assembly 254 is permitted, yetfast or sudden moves are resisted (in proportion to the square of thevelocity, as is known in the art). In a further variation, the pistonassembly 254 may be provided with an elastically compressible member ormaterial (not shown), either externally located between the pistonassembly 254 and an anchor 12 or internally within the piston fluidchamber. In still another variation, the piston assembly 254 may have afluid of mixed phases, either of same or different material, so that thepiston assembly 254 includes the compressibility of a gas form and theincompressibility of a liquid form, and the liquid and gas may beadjusted as desired.

As described, the piston assemblies 224 and 254 may be compressed onlyin their longitudinal directions, though they would have limitedflexibility in other directions. Accordingly, the piston assemblies 224,254 generally only permit flexure/compression in the anterior-posteriordirections. The piston assemblies 224, 254 may be calibrated so as toselect a desired amount of “stiffness” in their compression. If acompressible fluid were utilized, the “stiffness” may be variable (asopposed to linear based on Boyle's law). Additionally, the fluid may bea non-Newtonian fluid so that shear rate versus force is non-linear, ormay have a damper effect by using a fluid of high viscosity and/orinternal damper structure. The stiffness characteristics of differentpiston assemblies in the spinal stabilization systems may vary fromassembly to assembly so that, for instance, the stiffness between twovertebral levels may have a first set of characteristics, while thestiffness between two other vertebral levels may have a second set ofcharacteristics.

The above-noted reservoir may, alternatively, be located sub-cutaneouslyso that post-operative adjustment can be made without revision surgery.In some forms, separate valves may be provided on the piston assembliesfor increasing pressure and for decreasing pressure. Additionally, theabove-described keys or tools for adjusting the spanning structures orthe mechanical performance characteristics thereof may also be joinedwith the spanning structures and implanted such that non-surgicaladjustment of the keys or tools may be had via manipulation through theskin.

It should be noted that, as described, forms of the stabilization systemdescribed herein can be adjusted by a simple, relatively straightforwardrevision procedure, as described for the form of FIG. 6. The spanningportions described herein allow a continuous adjustment and selection(as opposed to an incremented selection based on rod diameter) of thestiffness or modulus of elasticity (or set of characteristics relatingthereto). Additionally, spanning portions extending between an inferiorvertebra and a second (medial) adjacent vertebra may have a firststiffness, while spanning portions extending between the medial vertebraand an adjacent superior vertebra may have a second stiffness orcharacteristics relating thereto.

A variety of forms of spanning structures are illustrated in FIGS.31A-31C. A spanning structure 270 may be constructed of various layersof material, two or more of which have differing linear moduli ofelasticity. The thickness of the layers may be selected to impart avarying spring equation to the spanning structure 270 over itslongitudinal length. For instance, a central core portion 272 may beformed of material with a first modulus of elasticity, and the centralcore portion may have a varying cross-sectional shape so that the springequation for the core portion 272 varies over its longitudinal length.In order to maintain a constant outer diameter to the spanning structure270, a layer 274 of constant outer diameter may be applied over the coreportion, the layer 274 having a varying inner diameter corresponding tothe outer diameter of the core portion 272. In this embodiment, thematerial of the layer portion 274 has an elastic modulus different fromthat of the core portion 272, and the materials and geometries of thecore and layer (or layers) are selected to control or provide a specificset of flexure/bending characteristics.

In another form, a spanning structure 280 may have a hollow core or bore282 of varying inner diameter. For instance, the bore 282 may have aconical shape (FIG. 22B), a double-frustum shape (FIG. 22C), or anothershape. The varying inner diameter allows for the bending of the spanningstructure 280 rod to be non-linear proportion to the force applied. Insome forms, the above-described scalloping 32, 44 may be formed on theinterior surface of the inner bore 282.

It should be noted that any of the above forms may be provided withshock absorbers or the like, such as at an interface between thespanning structures and the anchors. For instance, the spanningstructures and the anchors may be joined by an elastomeric or polymericcoupling.

In variations of the present invention, the effective bendingcharacteristics of spanning structures may be varied by varying theirgeometry, structure, and/or composition. For instance, a single (first)spanning portion may have a varying cross-section over its length,and/or the first spanning portion may have varying cross-section incomparison to a second spanning portion. In some forms, the spanningportions may be constructed as composite or layered member to impartdesired flexure characteristics, including varying the thickness or sizeof layers so that the flexure characteristics are non-linear.

Additional benefits of the systems and methods described herein includeimproving device fixation, and/or preventing unwanted contact betweendevices and patient anatomy (e.g. the patient's spinal cord). Thefurther use of methods described above, including the use of softwareanalytics, may further aid in determining screw placement andorientation to achieve the ideal screw placement and/or rod shape. Forexample, the use of various apparatus described herein to achievedesired screw placement and orientation in turn provides improvedalignment of a secondary device, such as a rod, with the screws heads.This benefit in turn allows the surgeon/user to achieve optimal sagittaland/or coronal alignment, which assists in rod placement and improvescorrection of the patient's anatomy.

While various embodiments of the present disclosure have been describedin detail, it is apparent that modifications and alterations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and alterations are withinthe scope and spirit of the present disclosure, as set forth in thefollowing claims. For further illustration, the information andmaterials supplied with the provisional and non-provisional patentapplications from which this application claims priority are expresslymade a part of this disclosure and incorporated by reference herein intheir entirety.

It is expressly understood that where the term “patient” has been usedto describe the various embodiments of the disclosure, the term shouldnot be construed as limiting in any way. For instance, a patient couldbe either a human patient or an animal patient, and the apparatus andmethods described herein apply equally to veterinary science as theywould to surgical procedures performed on human anatomy. The apparatusand methods described herein therefore have application beyond surgicalprocedures used by spinal surgeons, and the concepts may be applied toother types of “patients” and procedures without departing from thespirit of the present disclosure.

The foregoing discussion of the disclosure has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the disclosure to the form or forms disclosed herein. In theforegoing Detailed Description for example, various features of thedisclosure are grouped together in one or more embodiments for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimed disclosurerequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby incorporated into this Detailed Description, with eachclaim standing on its own as a separate preferred embodiment of thedisclosure.

The present inventions, in various embodiments, include components,methods, processes, systems and/or apparatuses substantially as depictedand described herein, including various embodiments, subcombinations,and subsets thereof. Those of skill in the art will understand how tomake and use the present inventions after understanding the presentdisclosure. The present inventions, in various embodiments, includeproviding devices and processes in the absence of items not depictedand/or described herein or in various embodiments hereof, including inthe absence of such items as may have been used in previous devices orprocesses, e.g., for improving performance, achieving ease and\orreducing cost of implementation.

Moreover, though the present disclosure has included description of oneor more embodiments and certain variations and modifications, othervariations and modifications are within the scope of the disclosure,e.g., as may be within the skill and knowledge of those in the art,after understanding the present disclosure. It is intended to obtainrights which include alternative embodiments to the extent permitted,including alternate, interchangeable and/or equivalent structures,functions, ranges or steps to those claimed, whether or not suchalternate, interchangeable and/or equivalent structures, functions,ranges or steps are disclosed herein, and without intending to publiclydedicate any patentable subject matter.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques that fallwithin the spirit and scope of the invention.

What is claimed is:
 1. A spinal stabilization system securable with aplurality of vertebrae, the system comprising: at least one anchorconfigured for attachment to at least two vertebrae; a plurality ofspanning structures extending between and securable with the at leastone anchor, each spanning structure having an adjustable bendingstiffness; wherein the plurality of spanning structures are arranged inlayers extending substantially the distance between the at least twovertebrae; wherein each of the spanning structures is adjusted to imparta different bending stiffness between its respective anchors; whereinthe bending stiffness of each of the spanning structures is adjustableafter being secured with the anchors; wherein the layers comprise anouter member of a first cross-sectional area and an inner portion of asecond cross-sectional area that is larger than the firstcross-sectional area; and wherein the outer member cross-sectional areais smaller than the inner portion cross-sectional area, the outer memberand inner portion being configured to be positioned substantially withinanother structural member, the combined members being retained with afirst end securable with a first vertebral body and a second endoperatively fixable with a second vertebral body.
 2. The system of claim1, wherein the bending stiffness of each of the spanning structures isadjustable in an orientation corresponding to the orientation of the atleast two vertebrae.
 3. The system of claim 1, wherein the bendingstiffness of each of the spanning structures is adjustable in anterior,posterior, lateral, and torsional modes.
 4. The system of claim 1,wherein the bending stiffness of each of the spanning structures isvariable in both longitudinal and transverse planes relative to the atleast two vertebrae.
 5. The system of claim 1, wherein the bendingstiffness of each of the spanning structures is adjustable by selectionof the material of the inner portion, and wherein the material of theinner portion is introducable after securing the outer member to the atleast one anchor.
 6. The system of claim 1, wherein the inner portion iscomprised of a plurality of components, and wherein the bendingstiffness of each of the spanning structures may also be adjusted byselecting a number of inner portion components disposed within the outermember.
 7. The system of claim 6, wherein the bending stiffness of eachof the spanning structures is adjustable by removal or addition of innerportion components.
 8. The system of claim 1, wherein the bendingstiffness of each of the spanning structures is adjustable byorientation of the inner portion relative to the outer member.
 9. Thesystem of claim 8, wherein at least one of the outer member and theinner portion has eccentrically positioned regions of reducedcross-sectional area,. wherein rotation of the regions provides adirection of decreased bending stiffness.
 10. The system of claim 1,wherein the inner portion is at least one helical coil springconcentrically located inside the outer member comprised of at least oneother helical coil spring.
 11. The system of claim 10, wherein thebending stiffness of each of the spanning structures is also adjustableby adjusting at least one physical characteristic of the one or morehelical coil springs.
 12. The system of claim 11, wherein the at leastone physical characteristic of the one or more helical coil springsincludes at least one of the number of coils, the diameter of the coils,and the length of the spring.
 13. The system of claim 10, wherein theone or more helical coil springs provide adjustablecompression/expansion stiffness to the plurality of spanning structures.14. The system of claim 10, wherein one of the helical coil springsprovides a compression stiffness and the other helical coil springprovides an expansion stiffness to the plurality of spanning structures.15. The system of claim 1, wherein the compression/expansion stiffnessof the each spanning structure is selectively adjustable.
 16. The systemof claim 1, wherein the bending stiffness of each spanning structure isadjustable via a percutaneous incision in a patient's skin.
 17. Thesystem of claim 1, wherein at least one spanning structure of theplurality thereof is adjustable via an end thereof.
 18. The system ofclaim 1, wherein at least one spanning structure of the pluralitythereof is adjustable via an implanted key or tool without an incision.